This invention relates to medical devices or components for such devices. More specifically, it relates to bioabsorbable elastomers fabricated into devices or components for devices suitable for medical applications.
The desirability of elastomeric materials for medical applications has been well established. For example, Thermoplast. Elastomers 3, Pap. Two-Day Semin., 3rd, pp. 68-71 (1991) discloses the fabrication of films and sheeting from copolyester elastomers for medical applications. These films can be used as transdermal patches for delivering bioactive agents through the surface of the skin, surgical wound dressings, I.V. site dressings, ostomy site dressings, and operating room garments. The copolyester elastomers are polymers with "hard" and "soft" segments. Their properties, such as flexibility, elasticity, and resistance to creep, can be tailored by varying the ratio of the hard and soft segments in the copolyester.
In addition to certain copolyesters which have elastomeric properties suitable for medical applications, polyurethane elastomers have also found acceptance within the medical community for numerous applications. This acceptance has led to the availability of Tecoflex.RTM. Aliphatic Polyurethanes for medical device applications. These elastomeric polyurethanes are prepared by reacting methylene bis(cyclohexyl) diisocyanate with poly(tetramethylene ether glycol). Some of the devices fabricated from these materials are intended primarily for implantation into the body. See the advertising brochure for TECOFLEX.RTM. Medical Grade Aliphatic Thermoplastic Polyurethanes from Thermedics, Inc.
While the commercial viability of elastomeric polymers for medical applications has been established, a need exists in the medical profession for certain properties which have not been met by the elastomeric polymers described above. For numerous applications, especially for those applications requiring a surgical device which is to be implanted in bodily tissue, the polymer from which the device is prepared must be bioabsorbable. In other words, the device must be capable of breaking down into small, non-toxic segments which can be metabolized or eliminated from the body without harm.
Unfortunately, although the elastomeric polymers described in the preceding references exhibit the requisite biocompatability, strength and processability, for numerous medical device applications, such elastomeric polymers are not absorbable in bodily tissue. Since these polymers are nonabsorbable in bodily tissue, surgical implants made from these elastomeric polymers would remain indefinitely within the bodily tissue, possibly causing adverse tissue reaction or other complications associated with the confinement of foreign matter in bodily tissue.
A large body of art has been created over many years, which focuses on the use of bioabsorbable polymers for numerous medical and surgical applications. As an example of this, the reader can review U.S. Pat. Nos. 5,133,739, 4,788,979 and 4,605,730. These patents teach the preparation of certain copolymer compositions of .epsilon.-caprolactone and glycolide for specific bioabsorbable medical applications. The copolymer compositions are described as particularly useful for the preparation of filaments suitable for use as sutures, and for use as coating compositions for coating the surface of sutures to improve tiedown properties. Although the copolymer compositions described in these references exhibit a combination of outstanding biological and physical properties which make such polymer compositions particularly well adapted for numerous surgical applications, such polymer compositions do not exhibit a desirable degree of elasticity. Therefore, these copolymers would not be desirable for use in medical applications requiring elastomeric properties.
A partial answer to the problem of developing elastomeric copolymers which are biocompatible and bioabsorbable in bodily tissue has been suggested in the art. Griipma et al., Polymer Bulletin 25, 327-333 (1991), describes a 50/50 mole per mole copolymer of L-lactide and .epsilon.-caprolactone. The copolymer is said to be elastomeric, and it degrades into non-toxic segments, so it is said to be useful for biomedical applications such as nerve guides. Similarly, U.S. Pat. Nos. 4,045,418 and 4,057,537 describe copolymers prepared from 75-85 parts by weight D,L-lactide and 25-15 parts of .epsilon.-caprolactone. The copolymers are stated to be easily moldable, thermoplastic elastomers, which are biodegradable to harmless substances. Additionally, the copolymers can be modified by replacing a portion of the lactide with glycolide, and thus preparing a terpolymer of lactide/glycolide/.epsilon.-caprolactone containing predominantly lactide.
While the elastomeric copolymers of lactide and .epsilon.-caprolactone (optionally including glycolide) have addressed the needs for certain medical device applications, such copolymers have a major drawback which has prevented their widespread use. Although the copolymers can be literally interpreted to be "bioabsorbable", the rate of absorption is so slow that it renders the copolymers practically useless for numerous medical applications. This is so because the predominant component of the copolymer, which is polylactide, absorbs very slowly in bodily tissue. The other primary component of the copolymer, polycaprolactone, absorbs even slower. In addition, lactide polymerizes faster than .epsilon.-caprolactone at 110.degree. C. so that when the copolymer is made, a segmented copolymer containing long segments of polylactide spaced between segments of polycaprolactone is produced. The segmented structure of the copolymer further lowers its bioabsorption rate. All of these factors create a copolymer whose components and morphology do not lend themselves to acceptable-bioabsorption rates for numerous medical applications.
In view of the deficiencies of the prior art, it would be highly desirable if medical devices or components for these devices could be fabricated from biocompatible polymers which exhibit the highly desired property of elasticity, without sacrificing mechanical properties, and yet also exhibit a rate of bioabsorbability which is fast enough for numerous medical device applications.